The invention relates to DC motors and, more particularly, to a process of limiting current surge of brush or brushless DC motors during severe voltage changes.
Brush and brushless DC motors have been used frequently in battery-supplied applications, such as automotive, electrical vehicle, forklift, etc. The battery voltage can change drastically when a large load is engaged, such as a starter motor, etc. A severe voltage change can cause a current surge in the motor since the current control loop is usually not fast enough to respond to the voltage change. As a result, the motor or power electronics may fail. FIG. 1 shows a typical current surge caused by a severe voltage change in a DC motor.
The motor current is controlled at a reference value before a voltage dip occurs. When the voltage dips, the current dips as well. Then, the current control loop starts increasing the PWM duty cycle to compensate for the current decrease. When the voltage starts rising, the current control loop is not fast enough to reduce the PWM duty cycle accordingly. Therefore, a current surge occurs due to the high voltage and high duty cycle. Improving the response time of the current control loop can help suppress current surges, but this improvement is limited since the current loop""s response time also depends on the sensing circuitry, motor parameters and the capability of a micro-controller.
Accordingly, there is a need to provide a voltage-based current limitation process that can respond to voltage changes quickly.
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by method, in a DC motor, of preventing a current surge during severe voltage changes. The method measures a voltage of the motor, compares the measured voltage to a set under-voltage limit and if the measured voltage is below the under voltage limit, stops the motor for a certain amount of time and then re-starts the motor. The method also calculates a pulse width modulated (PWM) duty cycle limit based on the measured voltage. An output PWM duty cycle, for controlling the motor, is compared to the PWM duty cycle limit and if the output PWM duty cycle is greater than the PWM duty cycle limit, the output PWM duty cycle is set to be the same as the PWM duty cycle limit.
In accordance with another aspect of the invention, a closed-loop control system for a DC motor is provided. The system includes a DC motor, a current sensor for determining a current of the motor, a voltage sensor for measuring a voltage of the motor, and a micro-controller. The micro-controller is constructed and arranged to (1) compare the determined current with a set point and output a pulse width modulated (PWM) duty cycle to control the motor, and (2) calculate a PWM duty cycle limit based on the measured voltage and compare the outputted PWM duty cycle to the PWM duty cycle limit such that when the outputted PWM duty cycle greater than the PWM duty cycle limit, the outputted PWM duty cycle is set to the PWM duty cycle limit. The micro-controller is also constructed and arranged to compare the measured voltage with an under-voltage limit and if the measured voltage is below the under-voltage limit, to stop the motor for a certain amount of time and then to re-start the motor.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.